In biological research and testing it is frequently necessary or desireable that cell cultures and the like be frozen so that they may be used at some future date. In order to be of use at some future date it is necessary that such cell cultures or the like be viable or at least contain a high percentage of viable cells with a good recovery rate when the cells are thawed. Unfortunately, the freezing of cells presents serious problems since improper freezing usually will result in damage to the cells thereby lowering the viability and recovery rate of the cells in the population.
This damage to the cells is caused by too rapid freezing of the cells that results in the formation of ice crystals in the cells because of the water present in the cells. This formation of ice crystals has been known to physically damage the cell walls. This ice crystal formation occurs in the temperature region from about minus five degrees Centigrade to about minus twenty-five degrees Centigrade and hence the manner in which cell cultures are cooled in this temperature zone is highly critical. In order for the cells to survive when passing through this temperature zone it is necessary that the cells be cooled comparatively gradually. It has been reported previously that a minus one degree Centigrade per minute rate is theoretically optimal. This desired cooling rate has been very difficult to achieve. This is made even more difficult by the fact that the common laboratory cooling substance is liquid nitrogen which due to its extremely low temperature tends to cool the cells too rapidly which is not acceptable.
In the past equipment for freezing cell cultures and the like was very complex and expensive and was generally designed to be used for large scale freezing operations. Such equipment in some instances required the use of heating as well as cooling. This equipment is prohibitively expensive for small laboratory use and its size and complexity also make it unsuitable for such use. U.S. Pat. Nos. 4,107,937 and 4,485,641 illustrate such complex apparatus.
There have been other smaller units made available. One of these is illustrated in U.S. Pat. No. 4,377,077. With this apparatus, the cell cultures are placed in a liquid refrigerant and then the entire apparatus is placed in a separate freezer. This requirement for a refrigerant and the use of a separate freezer complicates the use of this apparatus and makes it difficult to use. U.S. Pat. No. 4,388,814 illustrates the use of apparatus for cooling specimens that can use a dewar-type flask which is desireable for small laboratory use. However, a thermally conductive wall must be installed to give proper results. This plus the need to use a temperature sensing device and a controller greatly complicates the apparatus and makes it unsuited for small laboratory use.
This invention overcomes the problems associated with previous biological freezing apparatus. The biological freezing apparatus invention is inexpensive to construct and is simple and easy to use. The invention is well suited to small laboratory use and can be readily used with unmodified dewars and the invention requires no use of a continuous variable controller device.